The present invention relates generally to cardiac monitoring, and more particularly, to a method and an apparatus for chronically monitoring a myocardial performance index based upon sensed ventricular pressure waveforms.
The assessment of cardiac performance, or the efficiency of the heart as a pump, is a useful tool for monitoring the progression of a patient's heart disease, as well as for automatically driving any drug and/or electrical therapies of the patient. Cardiac performance is also routinely monitored as part of active clinical interactions, such as in drug titration and in observation of treatment effects.
A decrease in cardiac performance will likely affect the systolic and diastolic phases of a cardiac cycle differently. For that reason, cardiac performance assessment techniques traditionally focused on either the systolic or diastolic functioning of the heart. Common systolic function indicators include ejection fraction (i.e., the portion of blood that is pumped out of a filled ventricle during a contraction), stroke volume (i.e., the amount of blood pumped by the left ventricle of the heart in one contraction), cardiac output (i.e., the amount of blood pumped by the heart per minute), and systolic time interval. Examples of diastolic function indicators include isovolumetric relaxation time interval, isovolumetric contraction time interval, tricuspid/mitral valve inflow velocity patterns, and pulmonary vein/aorta flow.
Prior literature has suggested that combining one or more of the systolic and diastolic function indicators into an algorithm may yield a more effective measurement of the global performance of the heart than achieved by looking solely at one specific timing interval.
One such global cardiac performance indicator is the myocardial performance index (MPI) defined as the sum of the isovolumetric contraction time and the isovolumetric relaxation time divided by the ejection time. Each event of the MPI has been correlated to cellular measures of contractility. The isovolumetric contraction time has been correlated with SERCA++ release, the isovolumetric relaxation with SERCA++ uptake, and the ejection time with state (pre-load and rate) for normalizing isovolumetric contraction and relaxation.
The MPI is derived using echocardiography measurements and has been proven to be a useful predictor of morbidity and mortality in cardiac diseases. Although echocardiography is a noninvasive procedure, and these measurements are easily available, its use in deriving the MPI is limited to applications where the patient is located at a facility with the appropriate equipment, such as a hospital or medical clinic. As such, any attempts to chronically monitor the MPI based upon these echocardiography measurements would be resource exhausting. Moreover, conventional echocardiography techniques make it difficult to evaluate myocardial performance on a beat-by-beat basis, again limiting the usefulness of the MPI.